The present invention relates to an information-recording medium in which information is recorded through radiation of an energy beam, and the information-recording medium, and more particularly a phase change optical disk or a rewritable optical disk such as an optical-magnetic disk. Further, the present invention relates to an information recording apparatus in which either reproduction or recording for the information recording medium is carried out in a superior manner.
In the description of the present invention, the information-recording medium is sometimes expressed as a phase change optical disk, an optical magnetic disk, or merely an optical disk. In addition, if the information-recording medium is a medium in which radiation of energy beam may generate heat and this heat may generate a certain variation in atomic arrangement or magnetic momentum to perform information recording operation, the present invention can be applied to this medium, so that the present invention may also provide a certain effect against information recording medium other than a disk-like information recording medium such as an optical card and the like.
In addition, although the energy beam is sometimes expressed as a laser beam or merely as a laser light, if the energy beam can generate heat on the information recording medium as described above, the present invention can be applied to the information recording medium in which information is generally recorded with energy beam such as an electron beam, an ion beam or the like that are not normally meant by light.
In the case of rewritable optical disk, there was provided in general a method in which some groove or land (groove shape) type steps are set on a plastic substrate as a laser beam tracking and information is recorded at the groove or land. However, in order to improve a density in recording (a narrow track pitch), a method for recording information at each of groove and land under utilization of the groove and land shapes has been developed in recent years. In this case, the protruded shape in the groove and land is called as a land and the notch shape of the groove and land is defined as a groove. In general, in the case that a track pitch in the information recording medium is changed into a narrow track pitch down to about 80% of a laser beam spot and information is recorded at both land and groove, a leakage of reproducing signal is produced from the adjacent track (either the groove against the land or the land against the groove). For example, when information recorded at the land is reproduced, the reproducing signal from information recorded at the groove is leaked to generate a problem that information recorded at the land cannot be accurately reproduced. The leakage of the reproduced signal from information recorded in the adjacent track is called as a cross-talk.
In order to solve this problem, it is well known in the art to provide a method in which a plurality of magnetic layers are arranged to act as a recording layer and information only at the high temperature section is reproduced under utilization of temperature distribution at the recording layer when the laser beam is radiated (the prior art 1: Japanese Patent Laid-Open No.Hei 8-249737, the prior art 2: Japanese Patent Laid-Open No.Hei 9-293286). In accordance with these methods, since a temperature at the recording layer in the adjacent track becomes a low temperature as compared with the temperature at the recording layer of the center track, no leakage of the reproducing signal from information recorded in the adjacent track is generated. However, such a recording system as above does not provide any sufficient measure against a method for restricting a phenomenon (a so-called cross-erase) in which information (a recording mark) recorded in the adjacent track (the adjacent groove at the time of recording at the land or the adjacent land at the time of recording at the groove) is erased. For example, although the prior art 1 has a heat sink layer with a high thermal conductivity at a side opposite to the laser beam incident side, a distance between the magnetic layer and the heat sink layer is short with 20 nm or less, so that heat is easily dispersed at the adjacent track through the heat sink layer. In addition, although the distance between the magnetic layer and the heat sink layer in the prior art 2 is relatively thick of 80 nm, there are provided three metal magnetic layers as the recoding layer (a functional thin film changing in atomic arrangement or changing in electronic state under application of radiation of the laser beam and a total film thickness of the three layers has a quite thick thickness of 145 nm. Due to this fact, it has been found that heat is dispersed within the recording layer to delete information recorded in the adjacent track.
Accordingly, it has become a substantial problem that a technology for reducing a cross-erase is developed.
However, a step (a groove depth) between a center of the groove and a center of the land in the groove and land shape is usually set to xcex/8, where xcex is a wavelength of laser forming a laser beam. A reason why this value is set consists in the fact that the largest tracking error signal can be attained when the groove depth is xcex/8. However, there sometimes occurs a case that the groove depth is xcex/7 or more and xcex/5 or less due to the fact that a land groove recording system has been developed recently (prior art 3: Japanese Patent Laid-Open No. Hei 6-338064). A feature of this system consists in the fact that a cross-talk from an adjacent track (a leakage of signal from an adjacent track) can be cancelled even if a track pitch is made to be narrow by about 60% of that of the laser beam spot. However, this recording system does not provide a sufficient measure about a method for restricting a phenomenon (a so-called cross-erase) for deleting a recording mark recorded in the adjacent track (the adjacent groove at the time of land recording or the adjacent land at the time of groove recording). For example, in the prior art 3, although GeSbTe phase change recording material with a low thermal conductivity as compared with metal is used as a recording layer and further the film thickness of the recording layer is 5 to 50 nm, so that it has been found to generate a problem that a distance between the recording layer and the heat sink layer (a reflective layer) has a low value of 18 nm to cause heat to be dispersed into the adjacent track through the heat sink layer at the time of recording of information and a cross-erase is easily generated.
For example, under the standard of DVD-RAM with 2.6 GB/screen, since laser with a laser wavelength (xcex) being 645 to 660 nm is metered with a lens having a number of aperture (NA) of lens of 0.6, it is possible to perform a recording and a reproduction under application of the laser beam spot of 0.97 to 0.99 xcexcm (0.9 xcex/NA). Due to this fact, it is also possible to cancel the cross-talk even in the case that the track pitch is set to 0.7 xcexcm or less. To the contrary, the track pitch under the standard of DVD-RAM with 2.6 GB/screen is set to 0.74 xcexcm. Its reason consists in the fact that a cross-erase is generated in the case that the track pitch is made narrower than this value.
Accordingly, development of a technology for reducing the cross-erase is a substantial problem.
Additionally, a film structure of rewritable type optical disk is a multi-interference structure in which a dielectric member protective layer (hereinafter called as an under protective layer) such as Si2N4, ZnSxe2x80x94SiO2, a recording layer represented by TbFeCo type magnetic film or a chalcogenide type phase change film such as GeSbTe and the like, a dielectric member protective layer similar to the under protective layer (hereinafter called as a heat sink control layer) are laminated in sequence on a transparent substrate made of the plastic material and further there is provided a metal reflection film (hereinafter called as a heat sink layer) made of Al alloy, Au alloy and the like. A feature of this structure consists in attaining a high carrier wave to noise ratio by insisting a variation in optical characteristic value in the recording layer and reflecting light through setting refraction rates of each of the under protective layer, the recording layer, the heat sink control layer and the heat sink layer and setting of the film thickness to a proper value.
A function of the heat sink layer consists in reflecting light passed through the under protective layer, the recording layer and the heat sink control layer and in returning it to the incident side. Accordingly, it is required that its reflectivity is optically high. However, in general, a thermal conductivity of high reflectivity metal such as Al, Au, Pd, Pt, Cu and Ag or the like is quite high, so that some problems are generated as follows.
In the case that the heat sink layer has a high thermal conductivity, it means that heat generated at the recording layer may easily be dispersed into the heat sink layer, so that temperature at the recording layer is hardly increased and a laser power required for recording operation is increased (the recording sensitivity is reduced) (a problem of recording sensitivity).
In order to solve the problems, it is well known in the art to provide a method in which the two heat sink layers are arranged, a thermal conductivity of the heat sink layer near the recording layer is set as a low thermal conductivity and the side of the heat sink layer is provided with a heat sink layer having a high relative thermal conductivity (the prior art 4: Japanese Patent Laid-Open No.Hei 3-272032).
However, since the reflectivity of the low thermal conductivity metal satisfying the thermal conditions was 60%, the low thermal conductivity metal film could not sufficiently satisfy the optical characteristic as the reflective film, resulting in that a signal encoding degree of the reproducing signal, CNR ratio (a carrier wave to noise ratio) and an entire reflectivity of the multiplex interference structure or the like were hardly set to sufficient higher values (a problem of low CNR).
In addition, in the case that the heat sink layer has a high thermal conductivity, there may occur a phenomenon (a cross-erase) that heat is easily dispersed into a direction of plane of the heat sink layer and under influence of this heat, information recorded at the adjacent track is deleted (a problem of the cross-erase).
In addition, when the recording film of the rewritable type optical disk is made of phase change recording material, these information recording media are comprised of a protective layer, a recording film such as GeSbTe, a protective layer and a reflection area of GeSbTe on the substrate, and the reflectivity at the crystalline state is higher than that of amorphous state. With such an arrangement as above, the absorbing rate at the recording film shows a higher value at its amorphous state. If an over-writing is carried out under this state, the recording mark under its amorphous state may absorb light more easily than its crystalline state, resulting in that its temperature may easily be increased, and a size of the mark newly recorded is increased more than its normal size and a certain strain is generated at the reproducing signal.
In order to prevent this phenomenon, a trial has been accomplished to increase an absorbing rate under a crystalline state at the recording film than that of its amorphous state. For example, there are present a case in which a relation between an absorbing rate under a crystalline state and an absorbing rate under an amorphous state is reversed under an arrangement of a quite thin Au reflective layer with a thickness of 10 nm (the prior art 5: Shingaku Technical Bulletin MR92-71, CPM92-148 (1992-12) P.37) or a case in which a relation between an absorbing rate under a crystalline state and an absorbing rate under an amorphous state is reversed under an arrangement of Si at a reflective layer with a thickness of 65 nm (the prior art 6: Shingaku Technical Bulletin MR93-53, CPM93-105 (1993-12) P.1).
However, since any of these methods has no heat sink layer of sufficient thickness, it is not possible to perform a fast cooling of the recording film after the recording film is melted. Due to this fact, there occurs a problem that the recording film is deteriorated after performing the re-writing operation for several times (a deterioration problem of recording film when the re-writing operation is performed by several times).
In addition, in order to solve the problems above, there is provided a method in which two reflective layers are arranged at a side of the recording film opposite to the laser beam incident side, the reflective layer near the recording film is applied as Si and the other reflection film (a heat sink layer) is applied as an Al alloy, thereby a relation between the absorbing rate under the crystalline state and the absorbing rate under the amorphous state is reversed (the prior art 7: Proceeding of International Symposium on Optical Memory 1995, pp 151-152). Although this method is a superior one, reversing of the absorbing rate cannot be carried out sufficiently; since a thickness of the Si thin film must be limited to 50 nm to 100 nm in order to attain an optical superior multi interference structure, so that a degree of freedom in heat design is reduced; in addition, since there is provided a sufficient heat sink layer (a protective layer present between the recording layer and Si thin film), heat is dispersed to the adjacent track through the Si thin film having a high thermal conductivity and a heat sink layer (Al alloy) to generate the cross-erase. Further, since the semiconductor film such as Si or the like has normally a low film-forming rate, its productivity is not superior and it shows a certain problem in its production.
As described above in detail, when a high-density recording is carried out, various kinds of problems may occur. In particular, in the case that a high-density recording is carried out with a track pitch being less than 70% or less, a technology for remarkably reducing the cross-erase is an essential one.
Although all the methods were superior methods, a high-density recording to have the track pitch of 70% or less of the laser spot diameter was not sufficiently assumed such that the track pitch becomes 70% or less of the laser beam spot diameter, resulting in that all these methods could not reduce the cross-erase sufficiently. In addition, it is apparent that the structure where the cross-talk may easily occur is a structure in which a flow of heat from the land to the groove or from the groove to the land is substantially high. Further, in the case that the flow of heat between the land and the groove is high, there occurs a problem that the laser power required for the recording operation is different for the case of recording at the land and for the case of recording at the groove, respectively, when information is recorded at both land and groove as described later.
It is an object of the present invention to provide an information recording medium in which CNR, an over-writing characteristic and a recording sensitivity are not reduced even under a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter and further no cross-erase is produced.
Further, it is another object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR, an over-writing characteristic and a recording sensitivity and to realize an information recording medium of high recording density corresponding to an information recording and reproducing apparatus using a low-cost semiconductor laser.
Further, it is another object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR and a recording sensitivity and in particular to provide an information recording medium in which a superior over-writing characteristic is realized even under a high-density recording where a recording mark length may become a half or less of an energy beam spot diameter.
Further, it is a further object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR, an over-writing characteristic and a recording sensitivity and to provide an information recording medium in which a reproducing signal is not deteriorated even after many times of re-writing operation of about 100,000 times.
Further, it is a still further object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR, an over-writing characteristic and a recording sensitivity and further to provide an information recording medium in which no difference is produced in a recording sensitivity between a case having information recorded at the groove and a case having information recorded at the groove.
In order to solve the problems in the prior art described above and to accomplish the objects of the present invention, it is satisfactory to apply the following information-recording medium.
(1) An information-recording medium in which an atomic arrangement is changed under radiation of energy beam moved in a relative manner and/or information (a recording mark) is recorded under changing of electronic state, there are provided a plurality of information recording tracks in parallel with the relative moving directions, there is provided a step between the information recording tracks, one or more kinds of information recording thin films are provided as recording layers, sides of the recording layers opposite to the energy beam incident side are provided with one or more kinds of heat sink control layers and a heat sink layer having different composition against the heat sink control layer, a sum of film thicknesses of the more than one kind of recording layer is equal to or less than a step between information recording tracks, and a sum of film thicknesses of the more than one kind of heat sink control layer is equal to or more than a step between the information-recording tracks, thereby these features realize an information recording medium in which reduction of cross-erase, high CNR and improvement of durability against deterioration caused by re-writings of many times and further even in the case that information is recorded at both land and groove, quality of the reproducing signal attained from the land and the groove is not substantially different. Further, even in the case of high-density recording in which the track pitch is 70% or less of the laser beam spot diameter, the narrow track pitch can be realized without reducing CNR, over-writing characteristic and recording sensitivity, and an information recording medium is realized in which no difference in recording sensitivity is produced under a case in which information is recorded at the land and another case in which information is recorded at the groove. In particular, in the case that a sum of film thicknesses of the more than one kind of recording layers is 5 nm or more and 20 nm or less, the information recording medium with high CNR can be attained due to the fact that an optical interference effect between the heat sink layer and the recording layer can be effectively utilized under an easy permeation of laser light through the recording layer. In addition, in the case that a sum of film thicknesses of the more than one kind of heat sink control layer is more than a step between information recording tracks and 300 nm or less and more preferably 70 nm or more and 150 nm or less, its productivity is improved in addition to the effect.
(2) The information-recording medium according to (1) above is made such that a step between the information recording tracks is a step of notch and protrusion shape (a groove shape) and each of adjacent notch (a groove section) and adjacent protrusion (a land section) is provided with an information recording track, thereby a narrow track pitch of the information recording track can be realized without increasing a cross-erase.
(3) The information recording medium according to (1) and (2) above in which the information recording tracks are arranged under a certain track pitch (a distance between the center lines of the information recording tracks) and the track pitch is 50% or more and 70% or less of the energy beam spot diameter; and/or
(4) The information recording medium according to (3) above is made such that the energy beam is a laser beam, and a track pitch is more than 5 xcex/NA and less than 0.6 xcex/NA, where xcex denotes a laser wavelength applied in the laser beam and NA denotes the number of apertures of a lens for forming the laser beam, wherein the highest effect of the present invention appears in the information recording medium.
(5) The information-recording medium according to (4) above is made such that the information-recording medium is provided with a transparent substrate at an energy beam incident side, the energy beam is a laser beam, wherein a step (a groove depth) between a center of the notch and a center of the protrusion of the notch and protrusion shapes is xcex/(7n) or more and xcex/(6.2n) or less, where xcex denotes a laser wavelength of the laser beam and (n) denotes a refractive index of the transparent substrate at the wavelength xcex, thereby not only the cross-talk can be reduced, but also the cross-erase can be reduced without reducing a recording sensitivity, CNR and durability against multi-rewriting operation.
(6) The information recording medium according to any one of (4) and (5) above is made such that the information-recording medium is provided with a transparent substrate at an energy beam incident side, the energy beam is a laser beam, wherein a laser wavelength of the laser beam is 630 to 660 nm, the number of apertures of a lens for forming the laser beam is 0.57 to 0.643, a refractive index of the transparent substrate in the wavelength range is 1.45 to 1.65, a track pitch is 0.615xc2x10.03 xcexcm and the groove depth is 59 to 67 nm, thereby it is possible to realize the information recording medium having a high recording density such as a DVD-RAM with a recording capacity of 4.7 GB.
(7) The information-recording medium according to any one of (1) and (2) above, wherein the one kind or more of heat sink control layers contains metal oxide, carbide, nitride, sulfide and selenide; and/or
(8) The information-recording medium according to (7) above, wherein at least one kind of heat sink control layer in one kind or more of the heat sink control layer contains an S element; and/or
(9) The information recording medium according to (8) above, wherein an amount of inclusion of S element is 5% or more and 50% or less, a phenomenon of inverse flow of heat from the heat sink layer to the recording layer can be reduced due to the fact that a thermal conductivity of the heat sink control layer can be reduced by 1 W/mK and the cross-erase can be reduced without reducing a recording sensitivity, CNR and durability against multi-rewriting operation.
(10) The information-recording medium according to any one of (1) and (2) above, wherein there is provided the heat sink layer having a total amount of inclusion of Al, Cu, Ag, Au, Pt and Pd of more than 90%; and/or
(11) The information-recording medium according to (10) above, wherein there is provided the heat sink layer having an amount of inclusion of Al element of 97% or more; and/or
(12) The information-recording medium according to (10), wherein a film thickness of the heat sink layer is 30 nm or more and 300 nm or less and the cross-erase can be reduced without reducing a recording sensitivity, CNR and durability against multi re-writing operation during recording in which the recording layer is not only for a phase changed recording layer having as its major constituents Ge, Sb, Te, In, and Ag or the like, but also for an optical magnetic recording layer having as its major constituents Tb, Fe, Co, Dy and Gd or the like, for example.
(13) The information-recording medium according to any one of (1) to (12) above, wherein recording of information (a recording mark) is carried out under a changing from a crystalline state to amorphous state and/or from an amorphous state to crystalline state as the changing of atomic arrangement is performed, a narrow track pitch can be realized without reducing CNR, over-writing characteristic and recording sensitivity even under a high-density recording having a track pitch less than 70% of the laser beam spot diameter and it is possible to realize the information recording medium having a superior compatibility with the exclusive reproducing optical disk (for example, DVD-ROM) such as DVD-RAM, DVD-RW and DVD-R, for example.
(14) The information recording medium according to (13) above, wherein a rate of inclusion of S element is lower than a rate of inclusion of S element present in at least one kind of the heat sink control layer in the heat sink control layers and there is provided a thermal buffer present between the heat sink control layer and the heat sink layer; and/or
(15) The information recording medium according to (14) above, wherein thermal buffer layer is of a mixture material of metal, metal oxide, metal nitride and metal carbide; and/or
(16) The information recording medium according to (14) above in which a sum of film thicknesses of the heat sink control layer is more than a film thickness of thermal buffer layer, wherein a narrow track pitch can be realized without reducing CNR and a recording sensitivity under a high density recording where the track pitch is 70% or less of the laser beam spot diameter and a superior over-writing characteristic can be realized even under a high-density recording such that a recording mark length, in particular, may become a half size of an energy beam spot diameter.
(17) The information recording medium according to (14) above in which recording of information is carried out under recording of recording marks of a plurality of lengths and a length of the shortest recording mark in the recording marks of a plurality of lengths is 40% or more and 50% or less of an energy beam spot diameter, wherein the effect of the present invention remarkably appears.
(18) The information recording medium according to (13) above, wherein there are provided at least a plurality of heat sink control layers, a rate of inclusion of S element is lower than a rate of inclusion of S element present in at least one kind of heat sink control layer in other heat sink control layers and there is provided an interface layer present while being in contact with the recording layer, wherein a narrow track pitch can be realized without reducing CNR, an over-writing characteristic and a recording sensitivity and further a reproducing signal is not deteriorated after re-writing operation of many times of about 100,000 times even under a high density recording where the track pitch is 70% or less of the laser beam spot diameter.
In the present invention, although a plurality of thin films are laminated to realize a characteristic of each of the thin films, it is not necessary that each of the layers is strictly divided, if it is about 5 nm or less, for example, even if a composition ratio near an interface between each of the layers is changed continuously, the effect of the present invention is not lost. In addition, composition of each of the films in the present invention is indicated by an atomic %.
In addition, the energy beam spot diameter in the present invention is meant by a diameter of region where its intensity becomes 1/e2 or more of a central intensity of the beam spot. Further, the present invention provides an effect for improving a recording density in a direction in parallel with the information recording track (an effect where a superior over-writing characteristic is realized also when the record marks with a size of 50% or less of the energy beam diameter) and an effect for improving a recording density in respect to a vertical direction of the information recording track (an effect for restricting a cross-erase and a cross-talk even under a case in which the track pitch is 70% or less of the energy beam diameter). The energy beam spot diameter in the present invention is meant by a width of region where its intensity is more than 1/e2 or more of the central intensity of the beam spot in such a direction as one where each of the effects may appear in the case that a beam spot shape is an ellipse other than a circle, for example.
Further, definitions of each of the interface layer, the heat sink control layer, thermal buffer layer and the heat sink layer are set as follows. The interface layer is a layer which is present between the recording layer and the heat sink control layer to stabilize an interface at the recording layer, wherein an amount of inclusion of at least S element is smaller than an amount of inclusion of one kind of layer in the heat sink control layers and a film thickness is 2 to 10 nm. For example, if the composition of it is apparently different from that of the recording layer or the heat sink control layer when the surface of the recording layer is changed (nitriding, oxidization and the like) due to a certain processing at the surface of the recording layer, this is called as an interface layer.
In addition, the heat sink control layer is meant by a layer in which at least a film thickness is larger than 10 nm, it is present between the recording layer and the heat sink layer and as its major constituents, oxides, carbides, nitrides, sulfides and selenides for permeating the energy beam are applied and (k) in a plurality of refractive indexes (n), (k) at least at the wavelength of the energy beam is xe2x88x920.1 or more.
In addition, the heat sink layer is meant by a layer that is present at least between the heat sink control layer and the heat sink layer, (k) in a plurality of refractive indexes (n) (k) is lower than xe2x88x920.1 and a film thickness is 100 nm or less.